Rechargeable batteries are widely used power sources, for example, in medical devices, calculators, computers, appliances, and automobiles. Batteries consisting of solid state rechargeable electrochemical cells are of great interest because they allow important reductions in size and weight compared with the more traditional type of battery.
Solid state rechargeable electrochemical cells are well known in the art. Typically, these cells are constructed in layers composed of an alkali metal foil anode (negative electrode), an ionically conducting solid polymeric electrolyte separator, and a composite cathode (positive electrode). Terminals are attached to the anode and cathode thus forming an electrochemical cell which is suitable for generating an electrical current. The cell may be sealed in a gas and liquid impervious membrane from which the terminals protrude. These solid state cells are of great commercial and technical interest because they are capable of generating a relatively high current per unit area, and have a high current capacity. Use of solid state rechargeable electrochemical cells results in important weight reductions, for example, in automobiles, which leads to improved automobile fuel efficiency.
See, e.g. U.S. Pat. No. 4,935,317 (Fauteux et al, 1990) (discloses typical composite cathode compositions and use of lithium coated metal foil as an anode) and U.S. Pat. No. 4,990,413 (Lee et al, 1991) (discloses ionically conductive polymers suitable for cathode compositions).
Extensive efforts continue to be devoted to the development of low cost, highly efficient thin film solid state rechargeable electrochemical cells. The use of very thin lithium electrodes of thickness of 2 microns or less is a particularly attractive objective since reduction in the thickness of the lithium anode layer results in a reduction of the cell size and weight, and lower material cost. Additional cost savings can be realized through the successful development of manufacturing techniques designed to producer lithium electrode sheets in continuous lengths or in large batches. The very high reactivity of lithium, which makes it a material of choice for anodes, has the disadvantage that the surface of the lithium anode is readily degraded by atmospheric compounds such as moisture, oxygen and nitrogen. While the completed cell is sealed against atmospheric compounds, special precautions need to be taken to minimize or, most desirably, completely avoid atmospheric degradation of the lithium surface during manufacture of lithium anodes and electrochemical cell assembly. Effective protection of the lithium surface is even more critical for the contemplated very thin lithium anodes because these anodes contain minimal quantities of lithium metal.
Thin lithium anodes are desirable because lithium is not consumed during the use of these types of cells. A very thin lithium anode is as effective as a thick anode, to meet the electrochemical requirements of a cell. It is well known to those skilled in the art that very thin lithium foil is difficult and expensive to manufacture. Also, thin lithium foil, e.g., less than 20 microns thick, is too soft to have sufficient physical integrity for the production of cells and for strong connections with the required terminals. Lithium coated on a metal foil such as copper, nickel, titanium, stainless steel, chrome plated steel and nickel plated steel offers an effective compromise between the desire for a very thin lithium layer and the requirement for sufficient physical integrity of the anode.
Many methods for coating lithium onto metal substrates are know in the art. See, e.g., U.S. Pat. No. 3,551,184 (Dremann et al., 1970) which involves rubbing a heated metal substrate with a rod of lithium. U.S. Pat. No., 3,928,681 (Alaburda, 1975) discloses the coating of metal substrates as they are conveyed through an alkali metal melt. According to U.S. Pat. No. 4,824,746 (Belanger et al., 1989), lithium or lithium alloy is coated onto a metal substrate as the substrate is conveyed across a roller which is immersed in molten lithium or lithium alloy. U.S. Patent No. 5,169,446 (Koksbang et al., 1992) discloses the coating of lithium or other alkali metal on a metal substrate by contacting the substrate with molten lithium which is projected in the form of a standing wave. While these patents disclose methods for coating lithium on continuous lengths (such as sheet material that is handled in roll form) of a metal substrate such as foil, they all rely on thickness control by mechanical means. These mechanical means are thought to be insufficient to meet the requirement for highly reproducible thickness control of very thin coatings of alkali metals such as lithium of thickness less than 2 microns.
Reproducible thin coatings of lithium can be obtained by vacuum vapor deposition directly on very small stationery surfaces of solid state batteries. For example, Bates et al., "Rechargeable Thin-Film Lithium Microbatteries," Solid State Technology, pp.59-64 (July 1993) describes the preparation of thin film microbatteries wherein thin lithium anodes are prepared on solid inorganic glass electrolytes, such as lithium phosphorous oxynitride, by vacuum vapor deposition. The Bates batteries consist of successive layers of: cathode current collector, cathode, solid inorganic lithium electrolyte, lithium anode and a protective coating to protect the lithium anode. Takehara et al., "Thin Film Solid-State Lithium Batteries Prepared by Consecutive Vapor-Phase Process," J. Electrochem. Soc., 6:1574-1581 (1991), teaches the vacuum vapor deposition of thin lithium electrodes in the preparation of thin film solid state batteries with a total battery thickness of less than 20 microns. The effective surface area of these solid state lithium batteries was 49 mm.sup.2. The lithium anode was deposited on a 1 micron thick plasma-polymerized solid polymer electrolyte film.
U.S. Pat. No. 4,730,383 (Balkanski, 1988) discloses a process for producing a solid state battery wherein the lithium anode is prepared by vacuum vapor deposition on solid inorganic lithium electrolyte, or on a metal substrate. The Balkanski '383 microbattery consists of consecutive layers of metallic contact, intercalation compound cathode, solid inorganic electrolyte, lithium anode, and metallic contact layer. This battery is prepared in an apparatus consisting of three high-vacuum chambers connected together by two high-vacuum lock chambers. The cathode, electrolyte and anode are deposited on one another by vacuum vapor deposition in the three high-vacuum chambers, wherein each material is deposited on the battery in a separate chamber in order to form high purity layers of thickness less than 1 micron.
It is well known to deposit relatively non reactive metals on continuous lengths of metal foil. See, e.g., U.S. Pat. No. 3,990,390 (Plyshevsky et al., 1976) (vacuum vapor deposition of chrome, aluminum, nickel, copper and titanium on continuous lengths of metal foil); U.S. Pat. No. 2,665,227 (Clough et al., 1954) (vacuum vapor deposition of aluminum on continuous lengths of flexible metallic or plastic substrates); U.S. Pat. No. 3,044,438 (Osswald et al., 1962) (vacuum metallizing apparatus wherein a material such as aluminum is vaporized and subsequently deposited on a continuous sheet material such as plastic film). Typically, these vacuum vapor metallizing methods consist of continuously moving a foil or sheet substrate past a source of metal vapor in a vacuum chamber. Usually, the substrate foil is cooled by passing the inner surface of the foil over a cooling drum at the point where the metal is deposited on the outside surface of the foil. Generally, the metal which is to be evaporated is added in a continuous manner to the source of metal vapor.
Equipment for vacuum metallizing of continuous lengths of flexible substrate may consist of multiple chambers. (See, e.g., U.S. Pat. Nos. 5,254,169 to Wenk, 4,693,803 to Casey and Plyshevsky '390). These patents do not however, disclose methods or apparatus for vacuum vapor deposition of alkali metals such as lithium or highly corrosive metals with similar low melting temperatures, on continuous lengths of metal foil.
Regardless of the method used in coating a substrate with lithium or other alkali metal, steps must preferably be taken to prevent the lithium from reacting with the atmosphere.
Lithium is generally known to readily react with atmospheric compounds such as moisture, oxygen and nitrogen. Once the lithium surface has been chemically altered, it is a less effective surface for an electrochemical cell. It is thus important to protect the lithium surface from contamination or chemical attack during the manufacturing of these cells. This protection is particularly important where very thin lithium films (i.e., 2 micron thickness or less) are used as anodes.
The need to protect lithium anode surfaces is well known to those skilled in the art of making solid state rechargeable electrochemical cells. See, e.g., U.S. Pat. No. 4,502,903 (Bruder, 1985) disclosing a lithium anode film which is protected by laminating it with a conductive plastic substrate. The lithium surface is freshly prepared, e.g., by extrusion, in an argon atmosphere. A sheet of conductive plastic is laminated to one side of the lithium film under pressure and heat to obtain adhesion between the lithium surface and the conductive plastic which serves as an anode current collector. This method protects the outer side of the lithium film; but not the side which forms the cell's electrochemically active side.
U.S. Pat. Nos. 4,594,299 (Cook et al., 1986) and 4,818,643 (Cook et al., 1989) disclose a lithium anode film protected by a polymeric electrolyte.
Accordingly, the need exists for a method and apparatus to vacuum metallize alkali metal such as lithium on continuous lengths of metal substrate and protect the thin coating of alkali metal so obtained from reaction with a normal atmosphere.